Now that Mitt Romney has, in an important psychological sense, clinched the Republican nomination, the conversation is turning to the question of his running mate. And everyone seems to agree that Florida Senator Marco Rubio is a compelling option: young, telegenic, Hispanic and from a critical swing state. There’s just one problem — Rubio insists he doesn’t want to be Vice President.

It’s true that at times, he has offered something less than a completely airtight, LBJ-style “will not accept” statement. But in October he called the prospect “off the table” and said yes when asked if he was “ruling that out.” Just yesterday, he declared, “I’m not going to be Vice President.” To my ears, this is a solid notch more negative than the typical coyness of someone who covets the No. 2 slot but doesn’t want to appear overeager. By contrast, Ohio Senator Rob Portman, another likely Veep candidate, declined a similar chance recently from Politico to rule out the prospect, although he too downplayed his interest.

Of course, people have short attention spans, and it’s almost always possible to revise and update earlier statements about political intentions. Chris Christie, after all, called himself unready to be President and said there was “zero chance” he would run, but his party nearly begged him into the race anyway.

I suspect Rubio may understand that his political stock is overvalued, at least in this context. He’s an impressive young talent. But the assumption that Rubio can have a magic-wand effect over Latinos for the Republican ticket may be simplistic:

[T]he potential gains he could bring Republicans are hazy at best. Only 25 percent nationwide say Rubio would encourage them to vote Republican, compared to 47 percent who say it would make no difference. Nineteen percent say it would make them less likely to vote for Rubio, virtually negating the group who would be more likely to vote for him.

One reason for this is that Rubio’s position on immigration barely differs in substance from that of his party’s Arizona wing, even if he uses more-moderate rhetoric and offers piecemeal reforms like a much pared-down version of the Dream Act. Rubio’s ethnicity is also a thornier political question on the national stage than it is in Florida:

Another element ignored in many assessments of the issue is Rubio’s background as a Cuban-American. Whereas Cubans make up a plurality of Latino voters in Florida, 54 percent of the national Latino electorate is Mexican-American. Significant tension exists between the two groups, largely over the asylum status granted to Cubans in the U.S., which has in turn generated a class divide.

Those poll numbers aren’t set in stone, of course. They could change fast if Romney actually chooses Rubio and generates much media hype about the “Republican Obama” and if Rubio successfully positions himself as moderate on immigration.

But it’s far from a no-brainer. (Especially, by the way, if you consider his potentially heavy personal baggage.) If the benefits of adding Rubio to the ticket were clearer, I’d be more willing to discount his running-mate disavowals. In this case, I’m inclined to believe them.